The present invention relates to apparatus and techniques for improving the efficiency of a gas turbine engine, and more particularly, to such apparatus and techniques which recover pneumatic thermal energy from the engine and utilize such recovered energy to heat the engine fuel.
Modern conventional gas turbine engines, such as the one generally designated 10 in FIG. 1 include, inter alia, a fuel flow controlled by a fuel control valve 12. The engine 10 is provided with various engine bleeds, shown schematically in FIG. 1 as arrows A, B. The engine bleed of a conventional engine represents various points at which pneumatic energy developed by the engine is tapped, or bled, for particular purposes. In FIG. 1, Bleed A is representative of fan air; Bleed B.sub.1 is representative of mid-stage compressor air; and Bleed B.sub.2 is representative of compressor discharge air. Typical cruise temperature and pressure characteristics of such gas turbine engines for aircraft applications are: A=81.degree. F., 8 psia; B.sub.1 = 596.degree. F., 57 psia; and B.sub.2 =948.degree. F., 178 psia; respectively.
Various bleed lines may be employed for engine cooling purposes. Such engine bleed air is often utilized by support systems which are necessary in conventional aircraft. For example, such support systems may include an air supply system which provides airframe anti-icing air, engine cowl anti-icing air, and cabin environmental control system (ECS) air. This air supply system generally must output air within a predetermined temperature range, e.g., 425.degree. F. with anti-icing and 350.degree. F. without anti-icing. To maintain this temperature, one technique bleeds engine fan air (Bleed A) into an air-to-air heat exchanger. Compressor air (Bleed B) is also bled and directed through the air-to-air heat exchanger and cooled by the fan air to the appropriate temperature. The fan air is typically dumped overboard. This type of system is undesirable in that it results in a reduction in engine fuel efficiency due to unrecovered thermal energy from the compressor bleed air as well as the loss of engine fan air which would otherwise be available as engine thrust.
Further, it is well known that the availability of gas turbine fuel is limited to only a narrow spectrum of the distillate of already scarce petroleum crude. Thus, it would be desirable to provide a gas turbine engine which could efficiently operate on a wider spectrum of fuel distillates. One problem with such "wide spectrum" fuels is that such fuels exhibit a relatively higher freezing point, requiring some means for fuel tank heating.
Accordingly, it is a general object of this invention to provide a gas turbine engine having improved fuel efficiency.
Another object of this invention is to provide an energy recovery system for such a gas turbine engine which includes a support system in which some of the thermal energy made available to but not utilized by the support system is recovered and utilized to heat the engine fuel.
Another object of this invention is to provide such an energy recovery system in which some of the recovered energy is utilized to heat the fuel in the fuel tank.
Another object of this invention is to provide such an energy recovery system in which the recovered energy is selectively utilized to heat the engine fuel and/or the fuel in the fuel tank.
Another object of this invention is to provide such a gas turbine engine with reduced fan bleed.